The present invention relates to an apparatus and a method for measuring the length of such discontinuities as gaps, seams and cracks in electrically conductive parts or enclosures for determining their integrity with respect to such phenomena as radio frequency (RF) leakage and to mechanical strength characteristics.
2. Description of Related Art and Other Considerations
To secure electronic equipment from exterior electromagnetic radiation which could deleteriously affect its performance, such electronic equipment is conventionally packaged in a radiation secure enclosure. Conversely, it may also be necessary to protect the environment external to the electronic equipment from radiation generated from within the enclosure. Therefore, the enclosure must define a radiation secure environment. Such security is weakened by the presence of any discontinuities in the enclosure, such as slots, cracks, and open seams which will permit electromagnetic radiation to enter into or exit from the enclosure. It is, therefore, necessary to determine if such discontinuities exist and, if so, to locate and measure them.
Conventionally, enclosures have been tested by use of such instrumentation as time domain reflectometers, milliohm meters, devices employing magnaflux powders, feeler gages and transfer impedance measurement mechanisms. All of these instrumentation or techniques have one or more deficiencies.
The time domain reflectometer is a high band width instrument which obtains measurements at such high frequencies as to make the measurements susceptible to stray capacitance, which detracts from the accuracy of the test results. It is further a relatively expensive apparatus.
The milliohm meter cannot localize current paths and, therefore, is incapable of identifying the location of a gap or discontinuity. It only states that a gap exists, and then only if the gap is large.
In the magnaflux technique, ferromagnetic powder is placed on the enclosure to be tested, through which a large or high current is passed. The use of a high current requires that the enclosure have large paint-free contact areas. If such areas were previously painted, they would have to be later painted. This ferromagnetic powder also presents a potential contamination problem in that any residual powder must be removed. In addition, the high current utilized in the testing process poses safety problems.
Feeler gages constitute precisely dimensioned pieces of metal or the like, one or more of which is pushed into a suspected gap. Such gages are effective only on relatively large gaps and are ineffective if the gaps are not linear. Furthermore, the use of such a mechanical device creates a hazard of producing a gap in the enclosure where none had previously existed.
Transfer impedance measurement requires the use of microwave equipment and custom test jigs for each article. It cannot locate the gap, but only identifies if one exists. Further, it is not amenable to use in the field because it requires a shielded room.
Therefore, existing instrumentation and techniques have been found to be less than fully useful.